The invention relates to a rotor blade and a gas turbine having such a rotor blade.
Blades in steam and/or gas turbines are always subject to high mechanical, chemical and physical loads. As a result, attempts have been made for a long time to improve the rotor blades in particular with respect to their mechanical loading capacity such as stress distribution and resonance behavior. Thus, a rotor blade is shown in U.S. Pat. No. 6,976,826 B2 which is provided on the side of the blade tip with a pocket for accommodating a different material. The different material changes the weight distribution of the blade in the region of the blade tip and causes a change in the vibrational behavior and/or the resonance behavior. However, there is generally a risk that crack-prone edges and/or transitions will develop in the region of pocket, the result of which is that this solution requires expensive manufacturing. Furthermore, the material properties of the different type of material must be precisely defined. U.S. Pat. No. 6,755,986 B2 proposes a recess on the blade-root side to reduce the weight of a rotor blade. However, the disadvantage is that a change in the vibrational behavior and the stress progressions is almost impossible to achieve, because the rotor blade with its blade root and thus the recess are firmly clamped in the rotor. U.S. Pat. No. 4,595,340 shows a rotor blade with a recess on the blade-root side which extends through its platform into the blade pan, thereby making it possible to achieve modified vibrational behavior along with a reduction in weight. However, in this case there is a risk of weakening the blade pan.
In addition, providing single or multiple ribs as well as corresponding material thickenings to reduce vibrations is known in the case of platform overhangs that are elongated in the axial direction. However, these countermeasures mean excess weight, which may lead to cracks in the blade root. Furthermore, turbulence and thus flow losses may develop in particular as a result of the ribs.
The object of the present invention is creating a rotor blade that is optimized in terms of stress and vibrations, and which eliminates the aforementioned disadvantages and is simple to manufacture, as well as a gas turbine having such a rotor blade.
A rotor blade according to the invention for a turbomachine has a blade root for fastening on a rotor, a blade pan and a platform arranged between the blade root and the blade pan, which has at least one platform overhang. According to the invention, the platform overhang is provided with a variable transition radius in the transition area to the platform.
The variable transition radius allows the targeted adjustment of the rotor blade with respect to its vibrational behavior and its stress progressions. It is hereby possible to reduce an elevated stress concentration of the rotor blade in different blade regions and at the same time adjust the vibration on the platform overhang. Because the stress progressions in the rotor blade are taken into consideration or adjusted in a targeted manner, there is no weakening of the rotor blade, rather in fact a strengthening. In other words, the rotor blade according to the invention has an optimized center of gravity and an optimum weight with reduced stress concentration in specific regions with adjusted platform vibration and therefore an increased stiffness. The transition radius is configured directly during manufacturing of the rotor blade so that post-processing of the blade, as with the known filling of the pockets on the blade-tip side, is dispensed with. In addition, no ribs must be formed, which also simplifies manufacturing, for example a casting process. Furthermore, the rotor blade is made only of one material and not of a plurality of materials as is the case with the previously described pocket solution in the region of the blade tip.
In the case of one exemplary embodiment, the variable transition radius is configured on the blade-root side. There is hereby no impact on the flow progressions in the flow channel, in particular when a transition radius of the platform overhang on the blade-pan side is configured to be constant.
An especially effective influence on the vibrational behavior and the stress progressions and therefore the running behavior can be achieved, if the variable transition radius in platform regions with high loads is smaller than in platform regions with low loads. So, for example, in the case of one exemplary embodiment, the transition radius increases in particular in the rotational direction of the rotor. In doing so, the running behavior of the rotor blade can be further optimized if the rotor blade has a transition radius in the region of high loads that is reduced as compared to known rotor blades and a transition radius in the region of low loads that is increased as compared to known rotor blades.
Depending upon the relevant requirements, the transition radius may also be configured to be constant in sections. Thus, the transition radius is configured to be step-like in the case of one variant for example. It may likewise be designed to be constant only on the end side.
In the case in which the rotor blade has a further platform overhang, it may also be used to adjust the running behavior and be provided with a variable transition radius. It may likewise have a constant transition radius. Basically, the design of the transition radii on the platform overhangs depends on the respective requirements and may be selected to be variable. Thus, it is possible to provide different variable transition radii on both platform overhangs or even on only one of the platform overhangs.
A turbomachine according to the invention, in particular a gas turbine, has at least one rotor blade having a variable transition radius on the platform overhang. Such a gas turbine with a plurality of these types of rotor blades is characterized by a high level of running smoothness over its entire rpm range and is therefore suitable for aircraft engines in particular.
Other advantageous exemplary embodiments of the invention are the subject matter of further dependent claims.
A preferred exemplary embodiment of the invention will be explained in greater detail in the following on the basis of schematic representations.